Field of the Invention
The present invention relates to a printing apparatus, a conveyance apparatus, and a conveyance control method. Particularly, the present invention relates to a printing apparatus that stabilizes a back tension variation during, for example, roll paper conveyance by a conveyance roller, a conveyance apparatus, and a roll paper conveyance control method in the apparatus.
Description of the Related Art
There are printing apparatuses that use large paper having a size of A2 or more. A printing apparatus of this type often uses roll paper (a rolled portion where a sheet is wound will be referred to as roll paper, and a portion pulled out from the roll paper will be referred to as a sheet portion hereinafter) in addition to sheets. The sheet portion is pulled out from the roll paper by rotating a conveyance roller. However, since the roll paper is heavy in weight, a large force is necessary for pulling out the sheet portion. When only the driving force of a conveyance motor for driving the conveyance roller is used, the end portion of the roll paper is pulled, but the roll paper itself does not rotate because of its weight. Hence, the sheet may be torn. An apparatus has been developed, which includes a roll paper motor independently of the conveyance motor. The roll paper motor is driven together with the conveyance motor, thereby pulling out the sheet portion.
As a printing apparatus of this type, an apparatus disclosed in Japanese Patent Laid-Open No. 2009-263044 is known. This printing apparatus is of a type that intermittently conveys roll paper, and includes a conveyance roller, a roll paper motor, a conveyance motor, a tension measurement unit that measures a tension generated in the roll paper, and a motor control unit that controls driving of at least one of the roll paper motor and the conveyance motor based on the tension measurement result. In this arrangement, the motors are feedback-controlled based on the tension accurately measured by the tension measurement unit so as to decide the conveyance amount of the roll paper. This makes it possible to appropriately control the tension acting on the roll paper and prevent variation in the tension caused by a change in the diameter of the roll paper.
As an arrangement different from that described above, a printing apparatus disclosed in Japanese Patent Laid-Open No. 2007-203564 is also known. This printing apparatus includes a conveyance roller, a conveyance motor, a wind-off roller arranged at a position contactable with the outer surface of roll paper, a wind-off motor that rotates the wind-off roller, and a control unit that rotates the wind-off roller using the printing time and the conveyance time. In this arrangement, the roll paper is rotated in the conveyance direction by an amount corresponding to the conveyance amount necessary for the conveyance operation of the conveyance roller during the time from the end of the conveyance operation of the conveyance roller to the start of the next conveyance operation. This allows the roll paper to always have a slack so that the conveyance accuracy can be improved without any influence of the inertia of the roll paper.
There is known a conveyance apparatus that pulls out a sheet (to be referred to as a rolled portion hereinafter) wound into a roll and sandwiches the pulled out sheet between a conveyance roller and its associated roller, thereby conveying the sheet. A load is given to the axis portion of the rolled portion using a torque limiter or the like. The load is applied to the sheet portion as a back tension in a direction reverse to the conveyance direction so that an appropriate tension can be generated in the sheet between the rolled portion and the conveyance roller.
For example, Japanese Patent Laid-Open No. 9-164737 discloses an image printing apparatus including a conveyance apparatus provided with a remaining sheet detection unit that detects the remaining amount of a sheet wound into roll paper, and a back tension application unit capable of changing the torque of a load engaging with the axis of the roll paper.
In Japanese Patent Laid-Open No. 2009-263044, however, since the sheet portion is always given a tension, the sheet may slip on the conveyance roller due to the tension, and the actual conveyance amount may be smaller than that instructed by the control unit. This may affect the conveyance accuracy not a little.
To the contrary, according to an arrangement disclosed in Japanese Patent Laid-Open No. 2007-203564, the conveyance amount never becomes smaller because no tension is applied to the roll paper at all. On the other hand, skewed conveyance of the sheet portion may occur due to inappropriate setting by the user, a slightly nonuniform roller conveyance force in the direction of the roll paper width, or a slightly shifted parallelism between the roll center axis and the roller shaft.
To prevent the skewed conveyance, a method of applying a load by controlling the roll paper motor or using, for example, a torque limiter engaging with the axis of the roll paper is employed. When the load is applied to the roll paper as a back tension in a direction reverse to the conveyance direction, skewed conveyance of the roll paper is corrected by the back tension and the conveyance force so that wrinkles can be prevented.
However, in the arrangement of Japanese Patent Laid-Open No. 2007-203564, the back tension cannot be applied because of the absence of the load on the roll paper. Hence, it is difficult to correct skewed conveyance of the sheet portion.
In a printing apparatus for printing an image by serially scanning a carriage including a printhead, sheet conveyance and printing by carriage scan are alternately repeated, thereby printing an image on the entire sheet. In this case, the sheet conveyance amount at a time is equal to or smaller than the print length of the printhead. Let us examine one operation from the start to end of conveyance. There are the acceleration section, the steady section, and the deceleration section of the sheet, or only the acceleration section and the deceleration section. That is, the ratio of the acceleration and deceleration sections during the conveyance is high. In addition, to implement fast printing demanded of recent printing apparatuses, a higher conveyance velocity and a more abrupt acceleration/deceleration operation are required in some cases. Hence, the difference between the steady load set value and the load set value necessary at the time of acceleration/deceleration is supposed to be larger.
Under these circumstances, in the arrangement disclosed in Japanese Patent Laid-Open No. 9-164737 where the back tension variation caused by acceleration/deceleration of sheet conveyance is not taken into consideration, the sheet may slack because only the steady load torque corresponding to the remaining amount of the sheet is set. As a result, tension and slack repetitively occur in every conveyance operation. This may deteriorate the conveyance accuracy, resulting in poorer image quality.